| ( | X | + | 9 | ) | ÷ | ( | X | + | 100 | ) | = | 7 20 |
| Multiply both sides of the equation by: | ( | X | + | 100 | ) |
| ( | X | + | 9 | ) | = | 7 20 | ( | X | + | 100 | ) |
| X | + | 9 | = | 7 20 | ( | X | + | 100 | ) |
| X | + | 9 | = | 7 20 | X | + | 7 20 | × | 100 |
| X | + | 9 | = | 7 20 | X | + | 35 |
| X | − | 7 20 | X | = | 35 | − | 9 |
13 20 | X | = | 35 | − | 9 |
13 20 | X | = | 26 |
| X | = | 26 | ÷ | 13 20 |
| = | 26 | × | 20 13 |
| = | 2 | × | 20 |
| X | = | 40 |